Hitherto, as a data memory unit for computer, there is used a disc drive unit using magnetic disc as a recording medium.
As a magnetic disc used as a recording medium of the disc drive unit, there is used a magnetic disc in which magnetic film serving as a recording layer for information signals is formed on the surface of a disc substrate obtained by molding synthetic resin.
In disc drive units using magnetic disc of this kind as recording medium, the magnetic head is caused to scan, along inner and outer circumferential directions, the signal recording area of the magnetic disc rotated so that recording/reproduction of information signals is carried out. The magnetic discs used in this disc drive units are adapted so that high density has been realized along with increase in the recording capacity. In order to efficiently carry out recording and/or reproduction of information signals with respect to the magnetic discs in which high density has been realized and recording capacity has been increased as described above, it is necessary to improve transfer rate of information signals. In order to improve the transfer rate of information signals, it is necessary to increase the rotational speed of the magnetic disc, and to shorten the time required when the magnetic head seeks recording tracks of the magnetic disc.
In order to improve the seek speed of the magnetic head, there is used a disc drive unit such that dynamic pressure produced as the result of the fact that the magnetic disc rotates is utilized to produce air flow between the magnetic disc and the magnetic head to thereby carry out recording and/or reproduction of information signals in the state where the magnetic head is caused to float from the magnetic disc.
By using such flowing type magnetic head in this way, improvement in the transmission rate of information signals can be realized and the recording density of the magnetic disc can be improved. In addition, long life time of the magnetic disc and the magnetic head can be realized.
Meanwhile, in the magnetic head floating from the magnetic disc, in order to carry out stable recording and/or reproduction of information signals, the magnetic head is required to maintain stable attitude during recording and/or reproduction thus to scan on the magnetic disc with a fixed spacing being held from the magnetic disc. Namely, it is necessary to prevent that the spacing between the magnetic head and the magnetic disc is changed so that magnetic field strength across the magnetic head and the magnetic disc is fluctuated.
In view of the above, in the disc drive unit using the floating type magnetic head, the magnetic head is caused to float with a spacing of 1 .mu.m or less from the surface of the magnetic disc.
In the magnetic disc used in the disc drive unit adapted for allowing the magnetic head to float to carry out recording and/or reproduction of information signals as stated above, it is necessary to smooth, with high accuracy, the surface that the magnetic head scans. Since the magnetic head is permitted to float only by about 1 .mu.m or less from the surface of the magnetic disc, when very small uneven portion is waviness takes place at the surface of the magnetic disc, the magnetic head may collide with the rotating magnetic disc. As a result, there is also possibility that the magnetic head and/or the magnetic disc may be damaged.
In magnetic discs and/or optical discs using disc substrate obtained by molding synthetic resin, which are used as a recording medium of the disc drive unit, there have been proposed discs on which there are formed track pattern indicating recording tracks where information signals are recorded, and information signals and/or control signals are further recorded.
A disc substrate 101 constituting such magnetic disc is molded by using a metal mold unit 102 as shown in FIG. 1. This metal mold unit 102 comprises a first metal mold 104 serving as the fixed side and a second metal mold 105 serving as the movable side, which are adapted so that they are butted to each other to constitute a cavity 103 constituting the molding unit for molding the disc substrate 101. At the disc molding plane surface side constituting the cavity 103 of the first and second metal molds 104, 105, there are attached stampers 106, 107 for forming track pattern and/or uneven pattern corresponding to information signals and/or control signals on the both surfaces of the disc substrate 101 molded by this metal mold unit 102.
At the outer circumferential side of the second metal mold 105, there is assembled a ring-shaped outer circumferential side metal mold 108 which forms the outer circumferential surface of the disc substrate 101. Further, the first metal mold 104 is adapted so that there are disposed a sprue bush 109 in which a nozzle 109a for filling synthetic resin material in molten state constituting the disc substrate 101 into the cavity 103 is formed at the central portion thereof, and a stamper holder 110 fitted at the outer circumferential side of the sprue bush 109.
In addition, the second metal mold 105 is adapted so that there are disposed, at the central portion thereof, a punch 111 for forming a center hole at the disc substrate 101 molded within the cavity 103, and a ring-shaped eject member 112 within which the punch 111 is accommodated so that it can be advanced and withdrawn.
In this example, the metal mold unit 102 is adapted so that temperature adjustment circuits, etc. are respectively provided at the first and second metal molds 104 and 105 for the purpose of stably molding the disc substrate 101 under the constant temperature condition.
In order to mold the disc substrate 101 by the metal mold unit 102 provided with the configuration as described above, synthetic resin material in molten state is filled into the cavity 103 through the nozzle 109a of the sprue bush 109 in the mold clamped state where the first and second metal molds 104, 105 are butted to each other. The punch 111 is extruded into the cavity 103 in the state where the synthetic resin filled within the cavity 103 is semi-hardened to form a center hole of the disc substrate 101 to be molded. Thereafter, in the metal mold unit 102, the second metal mold 105 constituting the movable side is caused to undergo such an operation that it is spaced relative to the first metal mold 104 so that mold opening operation is carried out. Thus, molding operation of the disc substrate 101 is completed.
In this case, when mold opening operation is carried out, the disc substrate 101 affixed or attached to the second metal mold 105 side is pushed by an eject member 112. Thus, taking-out operation from the metal mold unit 102 of the molded disc substrate 101 is carried out.
At the surfaces which are respective principal surfaces of the disc substrate 101, uneven patterns formed on these stampers 106, 107 disposed within the metal mold unit 102 are transferred and formed by the stampers 106, 107 when the disc substrate 101 is molded.
Meanwhile, the stampers 106, 107 for forming uneven patterns on the disc substrate 101 are adapted so that the back side thrust to the smoothed molding plane surface sides of the first and second metal molds 104, 105 are polished in order to allow the disc substrate 101 to be molded so as to have a fixed thickness with high accuracy.
However, since the uneven patterns are formed on the surfaces of the stampers 106, 107, in the case where the back faces are caused to undergo abrasion processing, it is difficult to smooth them with high accuracy. As shown in FIG. 2, waviness would be produced. In FIG. 2, the ordinate indicates amplitude of the waviness and the abscissa indicates distance in the tangential direction of recording tracks formed in a concentrical form at the disc substrate 101.
As seen from FIG. 2, with respect to waviness taking place at the stampers 106, 107, when width of the head slider to which the magnetic head used in the disc drive unit is attached is assumed to be 2 mm, waviness of about 0.2 .mu.m takes place per width of the head slider.
Moreover, when the head slider is operated by 5000 .mu.m (5 mm) in the tangential direction of concentrical recording tracks at an arbitrary position in the radial direction of the disc substrate 101, relative large uneven waviness of about 200 nm as compared to the head slider having width caused to be 2 mm takes place in the thickness direction of the disc substrate 101 within the range of 5000 .mu.m as shown in FIG. 2.
When the disc substrate 101 is molded, pressure is applied to the stampers 106, 107 attached to the metal mold unit 102 by synthetic resin material filled into the cavity 103. By this pressure, waviness which have taken place at the back face sides appear at the surface sides where the uneven patterns are formed of the stampers 106, 107. As a result, the stampers 106, 107 would transfer undulations of the back faces to the disc substrate 101 to be molded or formed along with the uneven patterns.
The state where waviness taking place at the back faces of the stampers 106, 107 are transferred to the disc substrate 101 will now be described with reference to FIGS. 3 and 4.
In the state where no pressure is applied to the stampers 106, 107, waviness 113 which have taken place at positions designated at A, B and C of the back faces 106b of the respective stampers 106, 107 do not appear on the surfaces 106a of the respective stampers 106, 107 as shown in FIG. 3.
When the first and second metal molds 104, 105 are butted to each other so that mold clamping is carried out and synthetic resin material constituting the disc substrate 101 is filled into the cavity 103 as shown in FIG. 4, pressure of the synthetic resin material to be filled is applied to the stampers 106, 107. Thus, the stampers 106, 107 are deformed by this pressure and undulations 113 taking place at positions designated at A, B and C of the back face 106b appear on the surface 106a side facing to the cavity 103 side. As a result, such undulations would be transferred onto the disc substrate 101 to be molded or formed.
The state of the surface of the disc substrate 101 where undulations 113 of these stampers 106, 107 have been transferred will now be described with reference to FIG. 5.
In FIG. 5, the ordinate indicates amplitude of waviness 113 and the abscissae indicates distance in the tangential direction with respect to recording tracks formed in a concentrical form at the disc substrate 101.
As seen from FIG. 5, at the disc substrate 101, when the magnetic head scans the position spaced by distance of 4000 .mu.m in the tangential direction of recording tracks from an arbitrary position in the radial direction, projections of about 50 nm take place with respect to warp of the entirety of the disc substrate 101 within the range from 2000 .mu.m to 3000 .mu.m. Such projections are waviness in the projection form of the disc substrate 101 taking place resulting from the degree of surface roughness which indicate roughness of the back faces 106b of the stampers 106, 107. In addition, in FIG. 5, recessed portions in acute angle form of about 60 nm formed in the thickness direction of the disc substrate 101 are recessed patterns which constitute servo pattern and or track pattern, etc.
In this case, in the case where the magnetic head scans the signal recording surface of the magnetic disc constituted by using the above-described disc substrate 101 in the state where the head slider having width of the portion where the magnetic head is attached, which is caused to be 2 mm, is caused to float at height of interval of about 50 nm with respect to the disc average plane of the signal recording surface, if there exists projection 113a of about 50 nm with respect to warp of the entirety of the disc substrate 101 as shown in FIG. 5, the head slider 115 collides against the projection 113a, thus damaging the magnetic head or the magnetic disc.
The stampers 106, 107 fitted with respect to the above-described metal mold unit 102 come into contact with synthetic resin material of high temperature of 300.degree. C. or more at the time of molding of the disc substrate 101, and are caused to undergo thermal expansion in the radial direction. For this reason, uneven patterns formed so as to take concentrical complete round shape, which are provided for forming recording tracks of the disc substrate 101 formed at the stampers 106, 107, are distorted.
The state of the distortion of these stampers 106, 107 will now be described with reference to FIG. 6. In FIG. 6, broken lines indicate normal complete round recording track T.sub.1 of the stampers 106, 107, and solid line indicates, in a model form, track T.sub.2 in the case where the stampers 106, 107 are distorted and strained with respect to the center O.
Further, the stampers 106, 107 are formed by using nickel as material, wherein its weight is about 50 g which is approximately 1/500 of the first and second metal molds 104, 105 constituting the metal mold unit 102, and the thickness of the unit is about 1 mm. In addition, the respective stampers 106, 107 are attached to the first and second metal molds 104, 105 with their inner circumferential sides being supported. These stampers 106, 107 come into contact with high temperature molten synthetic resin material of 300.degree. C. or more filled within the cavity 103, so their temperatures suddenly rise. Thus, they are expanded in the radial direction. After molding of the disc substrate 101, these stampers are gradually cooled by the first and second metal molds 104, 105, so they are contracted. At this time, as shown in FIG. 6, the stampers 106, 107 are affected by the state of contact with the first and second metal molds 104, 105. As a result, the thermal expansion and the thermal contraction become uneven. Thus, the recording track T.sub.1 of complete round is deformed in the directions indicated by arrows in FIG. 6, so distortion takes place. Since the recording track T.sub.2 strained or deformed by uneven thermal expansion and thermal contraction is transferred onto the disc substrate 101 molded or formed by such stampers 106, 107, recording track formed on the disc substrate 101 would be strained or deformed.
Since the magnetic disc formed by using disc substrate 101 on which such strained or deformed recording tracks are formed is adapted so that since the servo pattern and/or the track pattern become eccentric, when such magnetic disc is loaded into the disc drive unit to carry out recording/reproduction of information signals, it becomes difficult that the magnetic head precisely scans recording tracks. Thus, recording/reproduction of information signals would fail to be carried out with satisfactory characteristic.
Moreover, coefficient of thermal expansion of synthetic resin material constituting the disc substrate 101 is 50.about.90.times.e.sup.-6 (1/deg). On the other hand, since main components of the stampers 106, 107 used for molding of the disc substrate 101 are nickel, the coefficient of thermal expansion is about 10.times.e.sup.-6 (1/deg). Since coefficients of thermal expansion of the disc substrate 101 and the stampers 106, 107 are different from each other, difference takes place between respective quantities of contraction in the cooling process where the disc substrate 101 and the stampers 106, 107 are cooled after the disc substrate 101 is molded. For this reason, in the disc substrate 101 which is molded body of solidified synthetic resin, stresses F.sub.1, F.sub.2 are respectively produced between the disc substrate 101 and the stampers 106, 107 by difference of coefficient of thermal expansion therebetween as shown in FIG. 7. As a result, the phenomenon that edge portion 122 of uneven pattern 121 formed on the disc substrate 101 is broken takes place.
Moreover, since the first and second metal molds 104, 105, the stampers 106, 107 and the disc substrate 101 respectively have different coefficients of thermal expansion, and quantity of thermal conduction and/or contraction quantity become uneven by the state of contact between the first and second metal molds 104, 105 and the stampers 106, 107, breakage of the edge portion 122 of the uneven pattern 121 is further complicated.
The state where, with respect to the disc substrate 101 molded by the above-described metal mold unit 102, the edge portion 121 of the uneven pattern 122 formed on the disc substrate 101 is broken by difference between thermal contraction factors of the disc substrate 101 and the first and second metal molds 104, 105 which are cooled and solidified) will now be described with reference to FIG. 8.
Synthetic resin material which forms the disc substrate 101, which is filled into the cavity 103 of the metal mold unit 102, has coefficient of thermal expansion of 12.times.e.sup.-6 (1/deg). The first and second metal molds 104, 105 have coefficient of thermal expansion of 12.times.e.sup.-6 (1/deg) in the case where they are formed by material of stainless system. Accordingly, the thermal contraction difference between the disc substrate 101 and the stampers 106, 107 which are cooled and solidified when the disc substrate 101 cooled and solidified at glass transition temperature (Tg).degree. C. is cooled so that its temperature reaches temperatures of the first and second metal molds 104, 105 is determined by (90-12).times.e.sup.-6, i.e., (Tg-temperature of metal mold).degree. C.
Further, with respect to the disc substrate 101 to be molded or formed, since the edge portion 122 of the uneven pattern 121 to be formed has mechanical strength smaller than that of uneven pattern 123 formed at the stampers 106, 107 as shown in FIG. 8, the edge portion 122 positioned at the central portion side of the disc substrate 101 is broken by the stampers 106, 107, so very small projection 122a takes place. In the case where such projections 122a is formed, when magnetic disc constituted by using this disc substrate 101 is loaded into the disc drive unit to carry out recording/reproduction of information signals, there is the possibility that the magnetic head may collide against the projection 122a of the disc substrate 101 so that the magnetic head is broken.
Further, the stampers 106, 107 fitted with respect to the above-described metal mold unit 102 is manufactured by experiencing mastering process for making up glass master block from glass base and electrocasting process for implementing electrocasting processing onto this glass master block. On the surface of the glass base of which surface is well polished, photoresist layer having uniform film thickness is formed by, e.g., spin coat process, etc. Namely, in the state where the glass base is mounted on the rotary table and is rotationally driven, photosensitive resist in liquid state is dropped on the central portion of the surface of the glass base. The photosensitive resist thus dropped is diffused at the surface of the glass base by centrifugal force to form photosensitive resist layer of uniform film thickness. Further, exposure corresponding to uneven pattern which constitutes various data recorded on the magnetic disc is carried out with respect to the photosensitive resist layer of the glass base. Thus, latent image of the uneven pattern is formed. This exposure is carried out by irradiation of laser. The latent image formed at the photosensitive resist layer of the glass base is actualized by development processing of the photosensitive resist layer. Thus, glass master block adapted so that the uneven pattern is formed on the surface thereof is manufactured.
Conductive film formation processing for forming metallic thin film by sputtering process or vacuum deposition process, etc. is implemented onto the surface of the glass master block. Further, from this glass master block, electrocasting processing is implemented thereto with the glass master block being as the electrode, whereby, e.g., nickel is deposited on its surface until a predetermined thickness is provided. Thus, making up of nickel master is carried out. The stampers 106, 107 are made up through mother made up by further implementing the electrocasting processing to this nickel master.
At the principal surfaces of stampers 106, 107 manufactured through process steps as described above, extremely precise and very small uneven portions are integrally formed in a concentrical form and/or radially in correspondence with uneven patterns corresponding to respective data recorded on the magnetic disc.
Meanwhile, at the metal mold unit 102 to which the stampers 106, 107 manufactured through process steps as described above are affixed, there is provided a temperature adjustment circuit for carrying out temperature adjustment of the first and second metal molds 104, 105. In this case, this temperature adjustment circuit does not serve to directly control temperatures of the stampers 106, 107 affixed on the first and second metal molds 104, 105. For this reason, precise temperature control of the stampers 106, 107 cannot be carried out. Therefore, there is the possibility that thermal deformation may be caused to take place at the time of molding of the disc substrate 101 so that degradation in accuracy of the uneven patterns transferred and formed with respect to the disc substrate 101 molded by these stampers 106, 107 is caused to take place, and warp or waviness, etc. may be caused to take place at the disc substrate 101.